2 Discrete Process ControlControl systems that operate on parameters and variables that change at discrete moments in time or at discrete events, usually binary (0 or 1, off or on, open or closed, etc.)Sensors Limit switch Contact/no contactPhoto-detector On/offTimer On/offActuators Motor On/offValve Open/closedClutch Engaged/not engaged

3 Categories of Discrete ControlLogic control – event-driven changesSequencing – time-driven changesLogic Control - a switching system whose output at any moment is determined exclusively by the values of inputsNo memoryNo operating characteristics that depend on timeAlso called combinational logic control

10 SequencingA switching system that uses internal timing devices to determine when to initiate changes in output variablesOutputs are usually generated “open loop”No feedback that control function is executedSequence of output signals is usually cyclical, as in a high production work cycleThe signals occur in the same repeated pattern within each regular cycleCommon sequencing devices:Timer – output switches on/off at preset timesCounter – counts electrical pulses and stores them

13 ExampleConstruct the ladder logic diagrams for (a) the NAND gate and (b) the NOR gate.Solution:If X1 or X2 remain open then C coil is unexcited and C contact remains closed, therefore Y is on.If X1 and X2 are closed then C coil is excited and C contact is opened and Y is off

14 SolutionIf X1 and X2 remain open then C coil is unexcited and C contact remains closed, therefore Y is on.If X1 or X2 are closed then C coil is excited and C contact is opened and Y is off

15 ExerciseThe production process may only be performed when the operator activates two spring activated safety switches. The switches have to be depressed and held closed together by the operator using both hands.(a) What is the truth table? (b) What is the Boolean logic expression? (c) What is the logic network diagram (in ISO symbol terminology)? (d) What is the ladder logic diagram?Assume X1 and X2 are the first and second switches and Y is the output

21 Advantages of PLCs Compared to Relay Control PanelsProgramming a PLC is easier than wiring a relay control panelPLC can be reprogrammedPLCs take less floor spaceGreater reliability, easier maintenancePLC can be connected to computer systems (CIM)PLCs can perform a greater variety of control functions

22 Typical PLC Operating CycleInput scan – inputs are read by processor and stored in memoryProgram scan – control program is executedInput values stored in memory are used in the control logic calculations to determine values of outputsOutput scan – output values are updated to agree with calculated valuesTime to perform the three steps (scan time) varies between 1 and 25 msec